Occurrence of Isomorphism and Quasi-Isostructurality in the Crystal Chemistry of Halogen-Substituted Phenylbenzimidamides.

The phenomenon of polymorphism and isostructurality, though conceptually distinct, is governed by the role of intermolecular interactions that are of importance in crystal packing. Here, we report the synthesis and structural characterization of mono- and di-substituted chloro/bromo phenyl benzimidamides to explore these phenomena. Extensive screening of the compounds resulted in (Z)-N'-(4-bromophenyl)benzimidamide existing as dimorphs, which, despite differences in crystal density and thermal stability, exhibit nearly isostructural packing, as confirmed by structure similarity analysis. Interaction energy calculations confirmed their isoenergetic nature and established them as being 'quasi-isostructural polymorphs', an intermediate state between polymorphism and isostructurality. Other halogen-substituted derivatives displayed similar unit cell parameters, and their structural similarity analysis confirmed the same. Three-dimensional isostructurality with a low dissimilarity index value, yielding isomorphous chloro/bromo pairs. The crystal packing analysis and intermolecular interaction energy calculations further revealed comparable interaction topologies and energetic contributions across these isostructural pairs. Quantitative investigation demonstrated consistent intermolecular interaction motifs, dominated by strong N…H hydrogen bonds, together with Cl/Br…H interactions. Together, these results provide deep insights into how subtle changes in the position of the same or different functional groups govern crystal packing and energetics in halogen-substituted molecules.